In semiconductor processing, a photolithographic mask (photomask) is used to pattern a radiation sensitive layer on a semiconductor substrate. Typically, the radiation sensitive layer is called a photoresist layer. A “photomask” is defined herein as the combination of a base and a patterning material. Typically, the base comprises a quartz or glass plate, which is transparent to the exposing radiation, and the patterning material lies on one side of the quartz plate and typically comprises chrome, aluminum, or gold, which is opaque to the exposing radiation. A stepping field is the portion of the patterning material that is used to pattern the photoresist layer.
The photoresist layer is sensitive to photomask defects, such as particles, for example. If a particle is present on a photomask within the stepping field when a photoresist layer is exposed using the photomask, the particle may cause a corresponding image to be formed in the patterned photoresist layer, which in turn may cause the device being fabricated to fail. If the stepping field of the photomask contains only one die, then all die that are fabricated on the semiconductor substrate could fail.
A pellicle is typically used to reduce the likelihood that particles migrate onto the stepping field of a photomask. A “pellicle” is defined herein to include a pellicle frame and a pellicle membrane. The pellicle membrane is typically a thin, flat, usually organic material, such as nitrocellulose or cellulose acetate, and it may be coated with one or more layers of fluoropolymers.
FIG. 1 illustrates a perspective view of prior art semiconductor photolithography equipment 1, including a prior art pellicle. The equipment 1 includes an illumination optical system 2, including a source 4 of photolithographic radiation 6. The equipment 1 further includes a photomask substrate 10 having on its upper surface 12 a photoresist or photolithographic pattern 14. The photomask comprises substrate 10 and photolithographic pattern 14.
To protect the photomask, a pellicle comprising pellicle frame 16 and pellicle membrane 20 is positioned a certain distance over the photomask. Pellicle frame 16 is typically a single-walled frame of metal, metal alloy, or plastic. Pellicle membrane 20 is typically fastened to pellicle frame 16 via an adhesive or adhesive tape (not shown).
A pellicle can protect a photomask from particles existing within the photolithography area. Photolithography is done in an ambient environment where particles are present, even in the cleanest of clean-room environments. Cleaning particles from the photomask can be difficult because of the relatively tight spacing between the photomask and the pellicle membrane. To properly clean a photomask and its pellicle often requires that they be removed from the photolithography area to a mask shop for several hours or even several days, at a significant expenditure of production resources. This can delay the production of semiconductors.
Therefore, there is a significant need for a pellicle that can be readily removed and replaced to facilitate cleaning of the photomask and pellicle membrane.
Further, the line dimensions of semiconductor devices are constantly shrinking. In order to achieve smaller patterning dimensions, photolithography must be carried out at increasingly shorter wavelengths. However, organic pellicle membranes cannot be used for shorter wavelengths. Radiation at such wavelengths is referred to as actinic, in that it can cause a chemical reaction in or near the pellicle membranes, causing them to deteriorate. For example, at approximately 200 nanometers (nm) and below a reaction occurs between the radiation and the air between the photomask and the pellicle membrane, producing ozone that breaks down an organic pellicle membrane. In some instances, the pellicle membrane becomes damaged during only one exposure to actinic radiation. In addition, at very short wavelengths, an organic pellicle is not transparent to the photolithographic radiation, because the pellicle has very high absorption at these wavelengths.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a significant need in the art for a pellicle device that adequately protects a photomask, and that does not deteriorate when the photomask is being exposed, as well as for methods of operating such a pellicle device.